Takes around 2 or 3 weeks to work into a new training program. Getting the proper pacing of the workout, weight used, style adjustments, etc.. And just as important, the mental approach to it. If not feeling comfortable with it after a few workouts, than drop the new routine.
Finding a program that seems to fit your needs, for the moment, than would suggest staying on it (feel free to make a few minor adjustments in exercise performance) for at least 6 to 8 weeks...12 weeks max. Try not staying on any given routine too long, else that routine becomes more of a habit than something that gives steady progress. See way too many guy's doing the same style workouts for years and pretty much looking the same and using the same weights, not gaining much in strength either. Good Luck
Excellent advice. Vary your training but when you do a routine give it time to work.